The recovery of alumina as alumina trihydrate from bauxite is accomplished in the Bayer process by digestion of the bauxite in a caustic medium at elevated temperatures and pressure. The resultant saturated sodium aluminate solution, know as the pregnant liquor, is part of a red mud slurry from which the inorganic, suspended solids are separated in two stages so as to prevent their contamination of the alumina product. In the primary settlement stage, flocculants are used to promote settlement of solids. The second stage is a pressurized filtration used to remove very fine suspended particles. The filtrate is then seeded to precipitate the alumina.
Bauxite contains organic impurities which are coextracted with the alumina during digestion and contaminate the liquor. Most of the organic impurities are high molecular weight compounds, such as humic acid, a portion of which decomposes to lower molecular weight compounds during digestion, producing a spectrum of organic salts dissolved in the liquor. Part of the organic impurities are color causing compounds called humates and impart a red-brown color to the liquor. Because the Bayer process involves continual recycling of the used caustic liquor to the digestion stage, the organic impurity content of the liquor increases with each recycle of the caustic liquor. If the humates are allowed to accumulate, the quality of alumina hydrate produced is affected and the colored material acts as a stabilizer for other dissolved organic impurities, such as oxalic acid.
Humate stabilization keeps low molecular weight organics in solution until over-saturated concentrations exist. Uncontrolled precipitation of the organics, particularly fine oxalates, then occurs during precipitation of the alumina hydrate thus interfering with satisfactory production of alumina hydrate. The oxalate causes nucleation of alumina hydrate with the resulting formation of very fine alumina hydrate that does not meet product standards. The oxalate may also precipitate on the surface of the alumina seed crystals and prevent the precipitation of alumina hydrate when desired. Oxalate also precipitates as scale on the walls of production vessels and thereby decreases heat transfer. Thus, humate removal is essential to the efficient running of the Bayer process.
The use of cationic polymers as flocculants for the Bayer process is reported in U.S. Pat. No. 4,578,255 and WO 87/00825. In the latter patent, the polymers were effective in separating the suspended solids, however, the samples treated were simulated bauxite using 20 g/l of China clay and 200 g/l of sodium hydroxide. Since native bauxite samples were not used, the substrate did not contain humates and therefore the problem solved was not comparable to the problem to be solved by this invention.
Bayer process liquors from native bauxite were treated with polydiallyldimethylammonium chloride with concentrations up to 200 mg/l with up to 50% reduction in absorption due to the humate in U.S. Pat. No. 4,578,255. Whereas this is a significant reduction of humate, the process fails to separate colloidal polydiallydimethylammonium chloride complexes that often form, pass through the filter and continually build up in the recycled Bayer liquors.